Exploration and production of hydrocarbons from subterranean formations often requires measurements of properties of the subterranean formations. For example, eddy currents induced in the subterranean formations may be used to provide measurements of the resistivity of the subterranean formations, which, in turn, may be used to determine where to drill to produce subterranean hydrocarbons. These induced eddy currents may themselves be measured by measuring small time-varying magnetic fields.
Measuring small time-varying magnetic fields with an induction coil requires the induction coil have a large effective cross-sectional area. In situations such as downhole in a borehole, there is a limit on the permissible diameter of the induction coil. Consequently, conventional approaches resort to increasing the number of turns of the induction coil to increase the effective cross-sectional area. However, as the number of turns of the induction coil increases, the resonant frequency of the induction coil decreases, limiting the usable bandwidth of the induction coil.
Reiderman, in U.S. Patent Application Publication Number 2006/0202699, published Sep. 14, 2006, appears to disclose a magnetic sensing assembly for measuring magnetic fields produced by a time-varying electric current generated in geological formations. The assembly comprises a soft magnetic core, a sensing coil wound on the core, and at least one miniature low noise magnetic field sensor disposed near an edge of the magnetic core. In one disclosed embodiment, the sensing induction coil comprises a plurality of sections disposed on the magnetic core, each section connected to its own low noise operational amplifier. However, such an assembly does not suppress unwanted coil resonances.